diff --git a/src/serialport_win.cpp b/src/serialport_win.cpp
index 7e79216a..7f8bac6f 100644
--- a/src/serialport_win.cpp
+++ b/src/serialport_win.cpp
@@ -326,18 +326,28 @@ bool IsClosingHandle(int fd) {
 
 void __stdcall WriteIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAPPED* ov) {
   WriteBaton* baton = static_cast<WriteBaton*>(ov->hEvent);
-  DWORD bytesWritten;
-  if (!GetOverlappedResult(int2handle(baton->fd), ov, &bytesWritten, TRUE)) {
-    errorCode = GetLastError();
-    ErrorCodeToString("Writing to COM port (GetOverlappedResult)", errorCode, baton->errorString);
+
+  if (errorCode) {
+    ErrorCodeToString("Writing to COM port (WriteIOCompletion)", errorCode, baton->errorString);
     baton->complete = true;
     return;
   }
-  if (bytesWritten) {
-    baton->offset += bytesWritten;
+
+  // bytesTransferred is already provided by the APC completion callback.
+  // Do NOT call GetOverlappedResult here — MSDN explicitly states:
+  //   "Do not use GetOverlappedResult for I/O operations that use
+  //    ReadFileEx or WriteFileEx completion routines."
+  // The overlapped's hEvent holds a WriteBaton pointer (not a Windows event
+  // handle), so GetOverlappedResult fails with ERROR_INVALID_HANDLE on
+  // drivers that inspect hEvent (e.g. usbser.sys used by USB serial chips).
+  if (bytesTransferred) {
+    baton->offset += bytesTransferred;
     if (baton->offset >= baton->bufferLength) {
       baton->complete = true;
     }
+  } else {
+    // Zero-byte completion with no error — avoid hanging in WriteThread.
+    baton->complete = true;
   }
 }
 
@@ -441,13 +451,15 @@ void __stdcall ReadIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAP
     return;
   }
 
+  // bytesTransferred is already provided by the APC completion callback.
+  // Do NOT call GetOverlappedResult here — MSDN explicitly states:
+  //   "Do not use GetOverlappedResult for I/O operations that use
+  //    ReadFileEx or WriteFileEx completion routines."
+  // The overlapped's hEvent holds a ReadBaton pointer (not a Windows event
+  // handle), so GetOverlappedResult fails with ERROR_INVALID_HANDLE on
+  // drivers that inspect hEvent (e.g. usbser.sys used by ESP32 native USB).
+
   DWORD lastError;
-  if (!GetOverlappedResult(int2handle(baton->fd), ov, &bytesTransferred, TRUE)) {
-    lastError = GetLastError();
-    ErrorCodeToString("Reading from COM port (GetOverlappedResult)", lastError, baton->errorString);
-    baton->complete = true;
-    return;
-  }
   if (bytesTransferred) {
     baton->bytesToRead -= bytesTransferred;
     baton->bytesRead += bytesTransferred;
@@ -455,7 +467,7 @@ void __stdcall ReadIOCompletion(DWORD errorCode, DWORD bytesTransferred, OVERLAP
   }
   if (!baton->bytesToRead) {
     baton->complete = true;
-    CloseHandle(ov->hEvent);
+    // Note: ov->hEvent is a baton pointer, not a Windows handle — do not CloseHandle
     return;
   }